"Yes, the moon," Luna smiled at him with pride. She then held out her hands, having ceased walking, to show him an illusion. Above her aloft palms came an image, a great creamy white circle that held its shape with some wavering as if the wind were blowing softly. Then in the image a figure appeared, rising from the bottom toward the top of the circle. It was dark and horse shaped, save for the pegasus wings and long horn from her head. It was a moving image of Luna herself raising the moon. As she showed this to Harry she explained, "I raise the moon, it is my task as princess. My sister raises the sun...however, I did live upon the moon....within the moon...."
Her voice then took on a new tone as she told the "fairytale" story of how she got there and became Nightmare Moon. It looked difficult for her to talk about, but she soldiered through.
"Once upon a time, in the magical land of Equestria, there were two regal sisters who ruled together, and created harmony for all the land. To do this, the eldest used her unicorn powers to raise the sun at dawn; the younger brought out the moon to begin the night. Thus, the two sisters maintained balance for their kingdom and their subjects, all the different types of ponies. But as time went on, the younger sister became resentful. The ponies relished and played in the day her elder sister brought forth, but shunned and slept through her beautiful night. One fateful day, the younger unicorn refused to lower the moon to make way for the dawn. The elder sister tried to reason with her, but the bitterness in the young one’s heart had transformed her into a wicked mare of darkness: Nightmare Moon."
The image became anew as the pony that had raised the moon now grew taller and cackled. Her coat grew dark, she revealed fangs with her laughter and took on a new, mature, villainous appearance. She wore shining battle armor and her mane seemed to exist forever.
"She vowed that she would shroud the land in eternal night. Reluctantly, the elder sister harnessed the most powerful magic known to ponydom: the Elements of Harmony. Using the magic of the Elements of Harmony, she defeated her younger sister, and banished her permanently in the moon. The elder sister took on responsibility for both sun and moon and harmony has been maintained in Equestria for generations since."
Luna then put her hands down and the image disappeared entirely, after showing Harry Nightmare Moon's defeat and transformation back into the precious and kind Luna.
"And yes, we have a factory for the weather. It creates rain, snow, clouds, tornados...etc...etc...the pegasus ponies are the weather ponies. They usher in new seasons as well as weather to make sure we always have bountiful crops and good weather."